There are several methods to defrost a ham. The refrigerator and cold water methods are the safest methods because the cooler temperatures discourage bacteria growth. To defrost a ham in the refrigerator, place the ham in a tray and put it in the refrigerator. To defrost a ham with cold water, place the ham in a leak-proof plastic bag, and submerge it in water that is less than 70 degrees Fahrenheit. Flip the ham regularly. Add cold water every 30 minutes to keep temperatures low.